The hydrogenation of aromatic nitro compounds over noble metal catalysts, such as Pd/C or Pt/C, is an important synthetic route to the corresponding aromatic amino compounds. However, the course of this reaction via a series of intermediates can be problematic. Hydroxylamines, which are formed as intermediates and which in the ideal course of a hydrogenation immediately react further, may accumulate in the reaction mixture with less active or unsuitable catalysts. Hydroxylamines are generally thermally unstable. The reaction mixture may therefore suddenly exothermically decompose. Moreover, undesired by-products are often formed starting from hydroxylamines. Furthermore, other groups present in the molecule, such as halogen substituents, may also be hydrogenated or cleaved off during the reaction with hydrogen and thus further undesired by-products may be formed.
It is known from the literature that an accumulation of unstable intermediates during the hydrogenation of nitro aromatic systems can be minimized by modification of noble metal catalysts with vanadium and phosphorus compounds in an oxidation state of <5. M. Studer and P. Baumeister in WO-A-96/36597 and P. Baumeister, H.-U. Blaser and M. Studer in Catalysis Letters 1997, 49, 219-222 thus describe the treatment of noble metal catalysts with various metals and state that treatment with vanadium compounds in particular is successful.
U. Siegrist, P. Baumeister, H.-U. Blaser and M. Studer in Chemical Industries (Dekker), 1998, 75 (Catalysis of Organic Reactions), 207-219 and P. Baumeister, U. Siegrist and M. Studer in EP-A-842920 describe an additional positive effect of phosphorus compounds in an oxidation state of <5.
Aufdenblatten, Rhony, Belser and Quittmann describe in WO-A-2011/036479 a process for the catalytic hydrogenation of aromatic or heteroaromatic nitro compounds to the corresponding amines over platinum catalysts, which were modified with a molybdenum and phosphorus compound in an oxidation state of <5.
Zhao, Chou and Chen describe in Industrial & Engineering Chemistry Research 2010, 49(4), 1669-1676 the use of W-modified Ni—Co—B catalysts for hydrogenating p-chloronitrobenzene, but not the use of platinum catalysts for hydrogenation.
Becher, Birkenstock, Waldau, Witt describe in DE-A-35 37 247 the use of modified Raney nickel catalysts for hydrogenating aromatic dinitro compounds to diamino compounds. Modifying metals used in this connection are, inter alia, Fe, Cr, Cu, Mo, Ta, W, V, Ti, Nb, Re, Ru, Zr, Hf. Platinum is also not used here.